Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!cbatt!osu-eddie!osu-cgrg!flip
From: flip@osu-cgrg.UUCP
Newsgroups: comp.lang.c
Subject: Re: Questionable Optimizations
Message-ID: <817@osu-cgrg.UUCP>
Date: Fri, 3-Apr-87 14:31:44 EST
Article-I.D.: osu-cgrg.817
Posted: Fri Apr  3 14:31:44 1987
Date-Received: Sun, 5-Apr-87 07:04:53 EST
References: <6943@ki4pv.UUCP>
Organization: Computer Graphics Research Group, Columbus OH
Lines: 57
Summary: an implementation...

In article <6943@ki4pv.UUCP>, tanner@ki4pv.UUCP writes:
> [...]
> The other is to allow them, and have some unwary sucker come
> complaining that his prog is giving him an APR trap or the local equiv.
> 
> It seems to me that we might want to consider explicitly specifying
> that under some circumstances, conforming compilers might be able to
> do these things, under user control.  A compiler could claim to
> support some (including improper) subset of the following OPT levels:
> 	0	=	don't do any opts; writes lousy code
> 	1	=	clean up code; don't do questionable re-writes
> 	2	=	do anything it can think of
> 
> A fair warning would be given: anyone using level 2 had better be
> sure of himself.  Any compiler is free to neglect to supply any of
> the levels, but must specify which it does support.  Any programmer
> who writes and immediately compiles level 2 deserves what he gets.
> 


The Convex C-1 vectorizing compiler does such a thing. Since vectorization
is a tricky thing and a compiler cant read your mind all of the time it 
allows a few things...

	1. compiler directives placed in line with the code. This allows
	    turning on and off certain optomizations, for example,
	    	
		/*$dir scalar */
		for(x=0; x<256; x++ ) {
			...
			
	    doesnt let the compiler try to vectorize a loop.
	    
	2. the -O0, -O1, and -O2 flags allow no, local and global scalar and
	    local and global vector and scalar optomization. This makes
	    things nice in your makefile for saying things like
	        
		OPT_LEVEL = 2
	        CFLAGS = -O$(OPT_LEVEL)
		
		...
		
	3. other flags such as the -F which performs "potentially unsafe
	    optomization"
	    
The combination of these things allows alot more control over how your code
is compiled. Possibly not the best approach, but it is most adequate for our
uses around here.
-- 

	Flip Phillips {ucbvax,decvax}!cbosg!osu-cgrg!flip
	Computer Graphics Research Group, The Ohio State University

	USnail: 1501 Neil Ave.,  Columbus, OH 43201

	"All I wanted was a Pepsi, and she wouldn't give it to me."
			-Suicidal Tendancies